The present invention relates to a process for determining the presence and/or quantity of H2S in subsoil and related apparatus. More particularly, the present invention relates to the determination of the presence and/or quantity of H2S in subsoil at different depths in relation to the surface and to an apparatus for implementing said process.
The field of the invention is the exploration and exploitation of subsoil resources, more particularly, but not exclusively, oil and geothermal type resources.
In this sector, the established practice of subsoil exploration is by means of drilling wells and the subsequent analysis of drilling mud brought to the surface. Drilling mud is a fluid product that functions to support the drilled hole, and lubricates and cools the auger when the well is drilled. This fluid product also functions to convey drilling debris to the surface, as well as volatile substances released while drilling.
At the surface, drilling debris arrives in a mixture with drilling fluids and possible volatile components dissolved therein. In the field of oil exploration, geological surveillance (mud logging) includes microscope analysis of drilling debris and chemical analysis (mainly of the chromatography type) of gases extracted from drilling mud. These analyses provide useful information both on the nature of the drilled rock layer and on features of oil resources that may exist in the subsoil. The gases present in the subsoil are both of the hydrocarbon and non-hydrocarbon type. Among the gases of the non-hydrocarbon type, hydrogen sulphide (H2S) is of particular importance because of its potential toxicity for humans. When the presence of H2S is revealed during drilling or its presence in the subsoil presumed, additives (scavengers) are fed into the drilling fluids and into mud whose purpose is that of reacting with gaseous H2S to form solid compounds to neutralize the potential harmful effects of this gas. On drilling sites, therefore, it is essential to be able to determine the presence of H2S in the subsoil with precision and as early as possible in order to guarantee maximum safety conditions for workers.
Determination of the quantity of H2S and its distribution in subsoil is also of fundamental importance for the purpose characterizing the geology of an exploration area to obtain useful information about potential oil resources. With state-of-the-art techniques and instruments, however, it is not possible to obtain sufficiently accurate information about the distribution and quantity of H2S in subsoil when a well is drilled.
In order to ensure safe working conditions, it is customary to detect the presence of H2S in the air around the drilling well by means of specific H2S sensors. The detection of H2S by sensors is generally reported to the workers by means of light and acoustic alarms. This type of detection can be inadequate from a safety viewpoint because workers are warned only when the risk is present or imminent. This type of detection also cannot be used to estimate the quantity of H2S effectively present in subsoil. The quantity of H2S detected in the air, in fact, is strongly influenced by dissolution and disassociation reactions which take place in the subsoil between the H2S in protonated form and drilling mud. Gaseous H2S in subsoil reacts with mud generally characterised by a basic pH forming the ionic species HS− and S2− which remain dissolved in the mud. Moreover, a fraction of H2S that remains in protonated gaseous form, and detected by sensors when it reaches the surface, is further reduced by the action of scavengers that are deliberately added to neutralise the H2S. The measurement of H2S in the air around the drilling well, therefore, considerably underestimates the quantity of gaseous H2S present in subsoil.
For similar reasons, traditional analyses which can be conducted on drilling mud to characterise subsoil also do not give accurate information about the quantity of H2S present.
The lack of measurement methods capable of determining the quantity of H2S present in the subsoil with sufficient accuracy makes it extremely difficult to investigate the distribution of this gas when drilling a well. To date, the only systems of direct measurement at depth which allow quantification of gaseous H2S in subsoil during drilling are those based on the use of special samplers (wireline formation testers) which allow spot sampling of drilled rock formation. These measurement systems, however, interrupt the drilling and entail high costs for the purchase and maintenance of instrumentation for sampling and sample analysis. As such, they have not to date been applied to a significant extent in this sector.
The object of the present invention, therefore, is to provide a process capable of determining the early presence and/or quantity of H2S in subsoil in order to improve the safety of wording conditions on the drill site.
A second object of the present invention is to provide a process capable of accurately determining the presence and/or quantity of H2S in subsoil by means of directly detecting gaseous H2S in drilling debris without interrupting the drilling process.
Moreover, a further object of the present invention is to provide a process capable of determining the H2S concentration profile in subsoil along the drilling line in order to obtain accurate information about the quantity and distribution of H2S at depth, and possibly characterise the area of exploration from a geological viewpoint.